Injection molding is a technology commonly used for high-volume manufacturing of parts made of meltable material, most commonly of parts made of thermoplastic polymers. During a repetitive injection molding process, a plastic resin, most often in the form of small beads or pellets, is introduced to an injection molding machine that melts the resin beads under heat, pressure, and shear. The now molten resin is forcefully injected into a mold cavity having a particular cavity shape. The injected plastic is held under pressure in the mold cavity, cooled, and then removed as a solidified part having a shape that essentially duplicates the cavity shape of the mold. The mold itself may have a single cavity or multiple cavities. Each cavity may be connected to a flow channel by a gate, which directs the flow of the molten resin into the cavity. A molded part may have one or more gates. It is common for large parts to have two, three, or more gates to reduce the flow distance the polymer must travel to fill the molded part. The one or multiple gates per cavity may be located anywhere on the part geometry, and possess any cross-section shape such as being essentially circular or be shaped with an aspect ratio of 1.1 or greater. Thus, a typical injection molding procedure comprises four basic operations: (1) heating the plastic in the injection molding machine to allow the plastic to flow under pressure; (2) injecting the melted plastic into a mold cavity or cavities defined between two mold halves that have been closed; (3) allowing the plastic to cool and harden in the cavity or cavities while under pressure; and (4) opening the mold halves and ejecting the part from the mold.
During the injection molding process, the molten plastic resin is injected into the mold cavity and the plastic resin is forcibly injected into the cavity by the injection molding machine until the plastic resin reaches the location in the cavity furthest from the gate. Thereafter, the plastic resin fills the cavity from the end back towards the gate. The resulting length and wall thickness of the part is a result of the shape of the mold cavity.
In some cases, it may be desirous to reduce the wall thickness of injected molded parts to reduce the plastic content, and thus cost, of the final part. Reducing wall thickness using a conventional high variable pressure injection molding process can be an expensive and a non-trivial task. In fact, conventional high variable pressure injection molding machines have a practical limit as to how thin walls of a part may be molded. Generally speaking, conventional high variable pressure injection molding machines cannot mold parts having a thinwall ratio (as defined by an L/T ratio set forth below) of greater than about 200. Furthermore, molding thinwall parts with thinwall ratios of more than 100 requires pressures at the high end of current capability and thus, presses that are capable of handling these high pressures.
When filling a thinwall part, the current industry practice is to fill the mold cavity at the highest possible rate the molding machine can achieve. This approach ensures that the mold cavity is filled before the polymer solidifies or “freezes off” in the mold, and provides the lowest possible cycle time since the polymer is exposed to the cooled mold cavity as quickly as possible. This approach has two drawbacks. The first is that to achieve very high filling velocities requires very high power loads, and this requires very expensive molding equipment. Further, most electric presses are unable to provide sufficient power to achieve these high filling rates, or require very complicated and expensive drive systems that substantially increase the cost of the molding equipment making them impractical economically.
The second drawback is that the high filling rates require very high pressures. These high pressures result in the need for very high clamping forces to hold the mold closed during filling, and these high clamping forces result in very expensive molding equipment. The high pressures also require injection mold cores that are made from very high strength materials, typically hardened tool steels. These high strength molds are also very expensive, and can be impractical economically for many molded components. Even with these substantial drawbacks, the need for thinwall injection molded components remains high, since these components use less polymer material to form the molded part, thereby resulting in material savings that more than offset the higher equipment costs. Further, some molded components require very thin design elements to perform properly, such as design elements that need to flex, or design elements that must mate with very small features of other design elements.
As a liquid plastic resin is introduced into an injection mold in a conventional high variable pressure injection molding process the material adjacent to the walls of the cavity, immediately begins to “freeze,” or solidify, or cure, or in the case of crystalline polymers the plastic resin begins to crystallize, because the liquid plastic resin cools to a temperature below the material's no flow temperature and portions of the liquid plastic become stationary. This frozen material adjacent to the walls of the mold narrows the flow path that the thermoplastic travels as it progresses to the end of the mold cavity. The thickness of the frozen material layer adjacent to the walls of the mold increases as the filling of the mold cavity progresses, this causes a progressive reduction in the cross sectional area the polymer must flow through to continue to fill the mold cavity. As material freezes, it also shrinks, pulling away from the mold cavity walls, which reduces effective cooling of the material by the mold cavity walls. As a result, conventional high variable pressure injection molding machines fill the mold cavity with plastic very quickly and then maintain a packing pressure to force the material outward against the sides of the mold cavity to enhance cooling and to maintain the correct shape of the molded part. Conventional high variable pressure injection molding machines typically have cycle times made up of about 10% injection time, about 50% packing time, and about 40% cooling time.
As plastic freezes in the mold cavity, conventional high variable pressure injection molding machines increase injection pressure (to maintain a substantially constant volumetric flow rate due to the smaller cross-sectional flow area). Increasing the pressure, however, has both cost and performance downsides. As the pressure required to mold the component increases, the molding equipment must be strong enough to withstand the additional pressure, which generally equates to being more expensive. A manufacturer may have to purchase new equipment to accommodate these increased pressures. Thus, a decrease in the wall thickness of a given part can result in significant capital expenses to accomplish the manufacturing via conventional injection molding techniques.
In an effort to avoid some of the drawbacks mentioned above, many conventional injection molding operations use shear-thinning plastic material to improve flow characteristics of the plastic material into the mold cavity. As the shear-thinning plastic material is injected into the mold cavity, shear forces generated between the plastic material and the mold cavity walls tend to reduce viscosity of the plastic material, thereby allowing the plastic material to flow more freely and easily into the mold cavity. As a result, it is possible to fill thinwall parts fast enough to avoid the material completely freezing off before the mold is completely filled.
Reduction in viscosity is directly related to the magnitude of shear forces generated between the plastic material and the feed system, and between the plastic material and the mold cavity wall. Thus, manufacturers of these shear-thinning materials and operators of injection molding systems have been driving injection molding pressures higher in an effort to increase shear, thus reducing viscosity. Typically, high output injection molding systems (e.g., class 101 and class 30 systems) inject the plastic material in to the mold cavity at melt pressures of typically 15,000 psi or more. Manufacturers of shear-thinning plastic material teach injection molding operators to inject the plastic material into the mold cavities above a minimum melt pressure. For example, polypropylene resin is typically processed at higher pressures. Press manufacturers and processing engineers typically recommend processing shear thinning polymers at the top end of the range, or significantly higher, to achieve maximum potential shear thinning, which is typically greater than 15,000 psi, to extract maximum thinning and better flow properties from the plastic material. Shear thinning thermoplastic polymers generally are processed at higher pressures. Even with the use of shear thinning plastics, a practical limit exists for high variable pressure injection molding of thin walled parts. This limit is currently in the range of thinwall parts having a thinwall ratio of 200 or more. Moreover, even parts having a thinwall ratio of between 100 and 200 may become cost prohibitive as these parts generally require injection pressures between about 15,000 psi and about 20,000 psi.
High production injection molding machines (i.e., class 101 and class 30 molding machines) that produce thinwalled consumer products exclusively use molds having a majority of the mold made from high hardness materials. High production injection molding machines typically experience 500,000 cycles per year or more. Industrial quality production molds must be designed to withstand at least 500,000 cycles per year, preferably more than 1,000,000 cycles per year, more preferably more than 5,000,000 cycles per year, and even more preferably more than 10,000,000 cycles per year. These machines have multi cavity molds and complex cooling systems to increase production rates. The high hardness materials are more capable of withstanding the repeated high pressure clamping operations than lower hardness materials. However, high hardness materials, such as most tool steels, have relatively low thermal conductivities, generally less than 20 BTU/HR FT ° F., which leads to long cooling times as heat is transferred through from the molten plastic material through the high hardness material.
Even with the ever increasing injection pressure ranges of existing high variable pressure injection molding machines, a practical limit remains of about 200 (L/T ratio) for molding thinwalled parts in conventional high (e.g., 20,000 psi) variable pressure injection molding machines and thinwall parts having a thinwall ratio of between about 100 and about 200 may be cost prohibitive for many manufacturers.
Injection Molding Systems Employing Variable-Position Molding Cavities
Increased complexity in an injection molded product's geometry or composition can require additional operations in the product's manufacture. For instance, products with multiple layers of plastic (e.g., different color), products with actuable parts (e.g., hingedly-connected or force-fit lids or caps) or products with integral details such as logo plates, require multiple injection mold shot cycles or other processing beyond a single injection mold shot cycle.
Many injection molded products, even those having complex geometry or composition, are conducive to being manufactured in multi-cavity molds that permit multiple quantities of the product to be molded simultaneously. Various developments have been made in an effort to increase manufacturing capacity and decrease cycle time of multi-cavity injection molding systems for products requiring multiple shots or other processing beyond a single injection mold shot cycle.
One type of system for molding parts requiring two or more shots of plastic involves a core back approach in which after a first injection molding shot is completed, a portion of a steel (or some other metallurgy, such as beryllium copper) mold immediately adjacent the mold cavity (or cavities) is partially pulled back, then a second injection molded shot is initiated. Once that second shot is completed, the mold is fully opened and the molded products may then be ejected.
Other systems for efficiently performing multiple injection molded shots or other operations to a multiple-cavity mold is to provide a mold having mold cavities that are variably-positionable. One such variably-positionable mold system employs a plate-mounted multi-cavity mold that is rotatable along a horizontal axis in a direction of a machine axis of the injection molding system. The injection molding system may have a first injection mold feed system (the first injection mold feed system including a first plastic resin source, a first screw, a first nozzle, and a first set of sprue gates for injection molding a shot of a first plastic material into a subset of the entire number of cavities of the multi-cavity mold), and a second injection mold feed system (the second injection mold feed system including a second plastic resin source, a second screw, a second nozzle, and a second set of sprue gates for injection molding a shot of a second plastic material into a subset of the entire number of cavities of the multi-cavity mold). The first injection mold feed system may be arranged such that the first set of sprue gates is alignable with mold cavities in an upper half of the mold, and the second mold feed system may be arranged such that the second set of sprue gates is alignable with mold cavities in a lower half of the mold. In use, the multi-cavity mold rotates from a first position, wherein a subset of the entire number of cavities of the multi-cavity mold is aligned with the first set of sprue gates of the first injection mold feed system to receive a shot of the first plastic material, to a second position, wherein that same subset of the entire number of cavities of the multi-cavity mold is brought into alignment with the second set of sprue gates of the second injection mold feed system to receive a shot of the second plastic material. The mold might rotate through 180° from the first position to the second position. While the second injection mold feed system is performing the shot of the second plastic material, the first injection mold feed system may simultaneously be performing a shot of the first plastic material on another subset of the entire number of cavities of the multi-cavity mold, which subset was out of alignment with the first set of sprue gates prior to rotation of the mold.
Alternately, in what is sometimes referred to as a “helicopter” shot injection mold system, the respective sprue gates of the first and second injection mold feed systems may be arranged relative to the mold such that the mold rotates through only 120° from the first position to the second position, and after the subset of the entire number of cavities of the multi-cavity mold receives the shot of the second plastic material, the mold rotates another 120° to a third position, at which a third injection mold feed system may perform a shot of a third plastic material, or alternately, an ejector may eject products from that subset of the entire number of cavities of the multi-cavity mold or some other processing operation could be performed. In such a system, each of the injection mold feed systems can continually run each time a subset of the entire number of cavities of the multi-cavity mold is brought into alignment with the sprue gates of that injection mold feed system, such that while one subset of the cavities of the multi-cavity mold is receiving its third shot of plastic material, another subset of the cavities is receiving its second shot of material, and yet another subset of the cavities is receiving its first shot of plastic material. In a helicopter mold system, the mold could stop at any desired number of locations around the horizontal axis about which it rotates.
Another variably-positionable mold system involves a mold rotatable about a vertical axis, the mold including a plurality of faces, each of which includes multiple mold cavities. For instance, a single cube mold system includes a four-faced mold, each of the four faces including multiple mold cavities. While each of the faces of a cube mold may each be square, having equal length and height, the faces may alternately be of any rectangular shape, such that the mold has four sides, but is not a true cube. The cube mold may be operated to rotate at 180° intervals so that opposed faces of the cube mold alternate between being in registration with a first injection side (at which a first injection shot is performed) and a second injection side (at which a second injection shot is performed). The cube mold may be operated to rotate at 90° intervals, so that operations might occur not only on opposing faces that are in alignment with first and second molding stations along a machine axis, but simultaneously on other faces of that same cube mold, further operations, such as a third injection mold shot, ejection, or cooling, can take place at the positions orthogonal to the first and second molding stations. In conventional high pressure cube mold systems where the operation being performed at a face of the cube mold orthogonal to the machine axis is a third mold operation, because that third injection shot takes place at high pressure, it is necessary to provide a strong clamping mechanism external to the cube that applies a mechanical wedge or lock that serves to wedge or trap together the plates in the face of the cube immediately facing that third molding station. While this clamping is not necessary for the faces of the cube mold along the machine axis, i.e. the axis along which the first and second molding stations are disposed in an opposing to one another, because as the system closes, the forces of the two opposing first and second injection molding stations serve to balance one another, since there is no off-setting mold opposing the third injection shot, the clamping pressure to secure the mold halves together on the third molding face must be provided by one or more additional mechanisms to achieve the wedge or lock. The mechanism(s) could be tied in to the press tie bars to provide a secure anchor point to generate the required clamping force. The capacity of the third mold shot is also lower than that of the first and second mold shots because of the pressures and limited ability to asymmetrically clamp the plates in the face exposed to that third molding station. It is recognized that the third shot can sequentially be introduced to a given set of mold cavities earlier in time than the second shot, since the third injection molding station is arranged intermediate the first and second injection molding stations, so the designation of “first”, “second”, or “third” as used herein is not intended to denote a particular order of operations, absent a specific additional indication that a particular of operations is intended.
The total cycle time for a variably-positionable mold system such as a single cube mold system is the total time for an entire revolution of the cube mold, including the duration of operations performed on cavities in the faces of the cube when the cube mold is stationary, as well as the time necessary to rotate the cube mold between the successive positions. In the case of a cube mold operated to rotate at 90° intervals, the total cycle time would include the time required for four rotations of the cube mold, plus four times the longest processing operation while the mold is stationary. The duration of the longest processing operation performed while the cube mold is stationary will define the cycle time on each mold face, because with operations being performed on different faces of the cube mold simultaneously, the cube mold must wait until the longest processing operation performed on cavities in any of the faces is completed before the cube mold may be rotated to advance the mold cavities in each of the faces to the next station about the perimeter of the cube mold. For instance, if a first molding station requires a longer period of time to perform an injection mold shot that a second molding station on an opposite side of the cube mold, such that the second molding station completes the second shot into mold cavities on the face of the cube mold immediately facing the second molding station before the first molding station completes the first shot into the mold cavities on the face of the cube mold immediately facing the first molding station, the cube mold is not rotated (i.e., advanced) until that first molding station completes the first shot.
Yet another variably-positionable mold system involves a double cube arrangement. In a double cube mold system, multi-cavity mold faces are provided on each side of two adjacent rotatable cube molds. This arrangement is particularly well-suited to automating assembly of multi-part products, since a set of first parts may be injection molded into cavities provided in a face of the first cube mold and a set of second parts may be injection molded into cavities provided in a face of the second cube mold. As the respective faces of the two cube molds are brought into registration with one another in a face-to-face relationship and the two cube molds are pressed together, each of the set of first parts can be forced into a mating relationship with a corresponding one of the set of second parts, such as in an interlocking or force-fit manner.
Variably-positionable injection mold systems have several benefits, such as their ability to increase productivity by increasing molding capacity per mold machine and reduce the time of overall product manufacture. In the case of a single- or double-cube injection molding system, there is also a significant potential reduction in energy costs resulting from reduced clamp tonnage necessary for the conventional high pressure injection molding performed within the mold system. In such a system, satisfaction of the highest clamp tonnage requirement of the two (or more) injection molding feed systems would provide the necessary clamp tonnage for an opposing injection molding feed system.
Because conventional multi-cavity injection molding systems operate at high pressures, it is well understood that it is necessary to employ product-specific mold inserts with high hardness, such as tool steels. These materials exhibit low thermal conductivity and have higher mass relative to lower-mass higher thermal conductivity materials, such as Aluminum. Due to the low thermal conductivity, in order to remove sufficient levels of heat from molded parts for completion of a mold cycle and for further processing or ejection, the use of extensive cooling channels in multi-cavity molds is prevalent. In the case of variably-positionable injection mold systems, the weight of the tool steel (or similar high hardness material) and the cooling channel requirements present limiting factors to the optimization of the dynamic features of the molds. For instance, in a single or dual cube mold system, cycle times must accommodate the time necessary to rotate the cube mold(s) from one position to the next. There are also significant energy requirements to actuate the heavy cubes between positions, as well as maintenance concerns to keep such actuation machinery in good working condition.